To increase the efficiency of gas turbine engines, a known approach is to raise the turbine operating temperature. As operating temperatures are increased, the thermal limits of certain engine components may be exceeded, resulting in material failure or, at the very least, reduced service life. In addition, the increased thermal expansion and contraction of these components adversely affects clearances and their interfitting relationships with other components of different thermal coefficients of expansion. Consequently, these components must be cooled to avoid potentially damaging consequences at elevated operating temperatures. It is common practice then to extract from the main airstream a portion of the compressed air at the output of the compressor for cooling purposes. So as not to unduly compromise the gain in engine operating efficiency achieved through higher operating temperatures, the amount of extracted cooling air should be held to a small percentage of the total main airstream. This requires that the cooling air be utilized with utmost efficiency in order to maintain the temperatures of these components within safe limits.
One gas turbine component which is subjected to extremely high temperatures is the shroud assembly which is located immediately downstream of the high pressure turbine nozzle. The shroud assembly closely surrounds the rotor of the high pressure turbine and thus defines the outer boundary of the extremely high temperature, energized gas stream flowing through the high pressure turbine. Adequate cooling of the shroud assembly is necessary to prevent part failure and to maintain proper clearance with the rotor blades of the high pressure turbine.
Furthermore, during engine operation the aft corners of the shroud are the hottest parts of the shroud. The aft corners are exposed to hot combustion gases that leak between adjacent shroud sections. Also, the aft corners are exposed to hot streaks, or regions of locally increased gas temperature as a result of uneven conditions around the circumference of the combustor. Excessive temperatures in the shroud can result in shroud distress, increased shroud leakage, and reduced engine performance.
A typical shroud assembly comprises a plurality of shroud hangers which are supported from the engine outer case and which in turn support a plurality of shroud sections. The shroud sections are held in place, in part, by an arcuate retainer or a plurality of arcuate retainers commonly referred to as C-clips. Pressurized cooling air is introduced through metering holes formed in the shroud hangers to baffle plenums disposed between the shroud hangers and the shroud sections. These baffle plenums are defined by pan-shaped baffles affixed to the hangers. Each baffle is provided with a plurality of perforations through which streams of air are directed into impingement cooling contact with the back or radially outer surface of the associated shroud section.
To achieve convection mode cooling, the shroud sections are provided with a plurality of passages extending therethrough. The baffle perforations are judiciously positioned such that the impingement cooling air contacting the shroud sections flows through the passages to provide convection cooling of the shroud sections. The convection cooling air exiting the passages then flows along the radially inner surfaces of the shroud sections to afford film cooling of the shroud. One element of the shroud assembly which does not receive direct cooling in this arrangement is the aforementioned C-clip. The result is that high operating temperatures can lead to overheating and possible failure of the C-clip. Accordingly, there is a need for a shroud assembly with improved cooling of the C-clip.